Pulse induction metal detector having high energy efficiency and sensitivity

ABSTRACT

A pulse induction metal detector having high efficiency and sensitivity. Within the scope of the invention is a high efficiency hand-held metal detector for detecting a metal object, comprising a coil and a source of current flow in the coil. The source is adapted to produce one or more pulses of current in the coil. Preferably, at least two of the pulses are spaced apart in time at least about 20 microseconds during which current flow in the coil is substantially constant. The metal detector also comprises a detecting circuit for detecting a response to the current pulses, and a sampling circuit for sampling the response at predetermined times at least during the time of one of the pulses.

FIELD OF THE INVENTION

[0001] The present invention relates generally to a pulse induction metal detector having high energy efficiency and sensitivity, especially a hobbyist type, hand-held, battery powered metal detector having these characteristics.

BACKGROUND OF THE INVENTION

[0002] Light-weight, hand-held metal detectors are popularly used by hobbyists and collectors to search for buried or otherwise hidden metal objects of value or particular interest, such as coins, jewelry, and artifacts of historical significance. Such metal detectors typically employ a transmit coil through which current flows, the current establishing time-varying magnetic fields that induce eddy currents in metal objects, and interact with any magnetic permeability of the metal object. These effects are detected in a receive coil, and at least the eddy current effects are indicative of the metal object, permitting its identification.

[0003] In addition to metal objects of interest, the ground itself typically contains metallic compounds, particularly compounds containing iron. The iron compounds in particular have a relatively high magnetic permeability that often masks the response of the detector to the metal objects in the ground. It is a problem in the metal detector art to eliminate all the ferrous mineral signals in the target volume of ground while retaining sufficient information to identify the metal objects.

[0004] Two classes of hand-held metal detectors have been developed in response to this problem. “Induction balance” detectors produce a continuous periodic interrogating signal. A substantially steady state response due to induced eddy currents in the target is measured during interrogation and is characterized by a magnitude and phase. The effect of ground mineralization is eliminated by adjusting the phase of detection so that it is insensitive to the phase angle of the ground. However, while the magnitude of the response for objects whose phase response is off this angle is thereby detected, the phase response itself is not, so one metal object cannot be distinguished from another.

[0005] As a solution to that problem, induction balance metal detectors have used what is known as “motion filtering,” which eliminates the effect of ground mineralization by making use of the fact that the ground mineralization is typically distributed over a larger area and is therefore relatively constant in space as compared to the metal objects of interest, such as coins. In motion filtering, the detector is moved over the ground and the rate of change of the response is determined, with the lower frequencies corresponding to the slower changing affects of ground mineralization being filtered out. With the effects of ground mineralization eliminated by motion filtering, the phase angle of the response can be used to discern one metal object from another.

[0006] More recently, multiple frequency induction balance has been developed to eliminate the need for motion filtering. In this method, two or more discrete frequencies are used providing two or more ground balanced signals which can be compared to permit discrimination between targets as well as ground nulling. Candy, U.S. Pat. No. 4,942,360 is representative of this practice. However, metal objects are not equally responsive to the different frequencies, so there is generally a single frequency that is optimum for discerning particular metal objects. Accordingly, a disadvantage of multiple frequency excitation is that the energy that would otherwise be used for the optimum single frequency is divided among several frequencies, with the result that the energy in each frequency is reduced and the depth of detection consequently suffers. This loss of energy efficiency can be a significant problem in battery powered metal detectors.

[0007] In induction balance detectors generally, a search head comprises at least two coils, one for transmitting and one for receiving. The coils are overlappingly arranged, or concentrically arranged along with a third, balancing coil, so that the effect of the transmit coil on the receive coil is nulled. Eddy currents induced in the target as a result of the target's response to the transmitted field, results in unbalancing the two coils, producing a response. The characteristic use of phase angles in induction balance detectors leads to this type being alternatively referred to in the art as “frequency domain” detectors.

[0008] The other of the two classes of metal detectors is known as “pulse induction,” in which a pulse is used to interrogate the target, and the target's response is measured after the interrogating pulse has ceased. To eliminate the major effect of ground mineralization, the method makes use of the fact that the response due to the permeability of the ground mineralization decays almost immediately. However, while the response due to magnetic permeability decays immediately, there is a remaining “magnetic remanence” or “viscous magnetic remanence” effect of the ground that, while not large, may remain in the response for some significant time after excitation. Moreover, the eddy currents induced in the target due to the presence of metal objects also decay after interrogation, and will decay quickly for small targets, so the signals are smaller than the signals obtained in an induction balance detector, making the pulse induction detectors generally less sensitive and less popular.

[0009] The use and analysis of a non-periodic time varying response in pulse induction detectors leads to this type also being referred to in the art as “time domain” detectors.

[0010] The pulse is critically damped with a damping resistor in order to ensure that the pulse stops as suddenly as possible without ringing, so that remaining eddy currents can be detected as soon as possible before they decay to such low levels that they are no longer useful. However, the damping resistor dissipates a significant amount of the energy used for interrogation. This energy loss is also particularly significant in a handheld, battery-powered metal detector.

[0011] Barringer, U.S. Pat. No. 3,105,934 proposes an airborne electromagnetic system for the remote detection of ore bodies utilizing pulses of short duration to induce transient polarization in the ore bodies. From a loop transmitter in an airplane normally flying at a survey flying height of 500 feet, 80 half-sine current pulses per second are produced and are asserted to thereby radiate a primary, pulsed electromagnetic field. A “bird” having three receiving coils is towed by the airplane, the receiving coils being responsive respectively to three components of secondary fields resolved with respect to the direction of flight. It is proposed that an ore body consisting of base metal values occurring in a massive sulphide deposit may be considered as a conductive sheet in which circulating current will be induced in the presence of the primary field, and that the time constant for decay of this current, when the primary field is absent or not time varying, is indicative of the conductivity of the sheet, which aids in the recognition of varied ore deposits.

[0012] Excepting the use of half-sine pulses, for which no reason is given, the system employs the standard pulse induction methodology with its attendant drawbacks. Moreover, the system is not amenable to hand-held use or battery power.

[0013] Accordingly, there is a need for a pulse induction metal detector having high energy efficiency and sensitivity, particularly where the capability of the metal detector to reject ground mineralization and to discern and distinguish different metal objects in a target is not sacrificed.

SUMMARY OF THE INVENTION

[0014] Disclosed is a pulse induction metal detector having high energy efficiency and sensitivity. Within the scope of the invention is a metal detector having a search coil, a source of current flow in the search coil for interrogating a target volume of ground, and a detecting circuit for receiving a response to said interrogating. A preferred method for detecting one or more unknown metal objects in the target volume of ground and for discriminating the metal objects from the ground comprises calibrating the metal detector and interrogating the target volume of ground. Calibration is performed with a calibrating volume of ground similar to the target of ground but in which the unknown metal objects are not substantially present. Calibration preferably comprises (a) providing that the current flow changes from an initial quiescent magnitude in a predetermined time-varying manner, (b) sampling the response of the detecting circuit to step (a) at least three distinct times within a sampling period to produce a respective three calibration data points, and (c) combining at least two of the three calibration data points in such manner as to produce a null result.

[0015] Interrogating includes (d) providing a first quiescent period during which the current flow is substantially unchanging from a first quiescent magnitude, (e) thereafter changing the amount of the current flow from the first quiescent magnitude in the manner of step (a), (f) providing a second quiescent period terminating step (e) during which the current flow is substantially unchanging from a second quiescent magnitude, (f) sampling the response of the detecting circuit to step (e) at least the three distinct times to produce a respective three target data points, and (g) combining the target data points in the manner of step (c) to produce at least one ground balanced target data point for comparison with preexisting data.

[0016] Also within the scope of the invention is a high efficiency hand-held metal detector for detecting a metal object, comprising a coil and a source of current flow in the coil. The source is preferably adapted to produce one or more half-sine pulses of current in the coil. The metal detector also comprises a detecting circuit for detecting a response to the current pulses, and a sampling circuit for sampling the response at predetermined times at least during the time of one of the pulses.

[0017] Also within the scope of the invention is a high efficiency hand-held metal detector for detecting a metal object, comprising a coil and a source of current flow in the coil. The source is adapted to produce two or more pulses of current in the coil. Preferably, at least two of the pulses are spaced apart in time at least about 20 microseconds during which current flow in the coil is substantially constant. The metal detector also comprises a detecting circuit for detecting a response to the current pulses, and a sampling circuit for sampling the response at predetermined times at least during the time of one of the pulses.

[0018] Therefore, it is an object of the present invention to provide a novel and improved pulse induction metal detector having high energy efficiency and sensitivity.

[0019] It is another object of the present invention to provide such a metal detector that provides for improved operating efficiency and economy without sacrificing the capability of the metal detector to reject ground mineralization.

[0020] It is still another object of the present invention to provide such a metal detector that provides for improved operating efficiency and economy without sacrificing the capability of the detector to discern and distinguish different metal objects in a target.

[0021] The foregoing and other objects, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a pictorial/schematic view of a metal detector according to the present invention.

[0023]FIG. 2 is a plot of the amplitude versus time of an electrical signal according to the present invention used as an interrogating signal in the metal detector of FIG. 1.

[0024]FIG. 3A is a plot of the current versus time for an exemplary waveform portion of the electrical signal of FIG. 2 according to the present invention.

[0025]FIG. 3B is a plot of the current versus time for an alternative example of a waveform portion of the electrical signal of FIG. 2 according to the present invention.

[0026]FIG. 3C is a plot of the current versus time for another alternative example of a waveform portion of the electrical signal of FIG. 2 according to the present invention.

[0027]FIG. 3D is a plot of the current versus time for yet another alternative example of a waveform portion of the electrical signal of FIG. 2 according to the present invention.

[0028]FIG. 3E is a plot of the current versus time for still another alternative example of a waveform portion of the electrical signal of FIG. 2 according to the present invention.

[0029]FIG. 4 is a plot of the magnitude of the voltage versus time for producing the current of FIG. 3B.

[0030]FIG. 5 is a schematic of a signal generator according to the present invention, for producing current waveforms according to FIGS. 3A-3E.

[0031]FIG. 6 is a plot of the current versus time for two current pulses that form a portion of a waveform according to the present invention, and the response of various target materials to the pulses used as an interrogating signal in the metal detector of FIG. 1.

[0032]FIG. 7 is a plot of the current versus time for a single half-sine current pulse excitation and a response for ferritic ground showing sample points.

[0033]FIG. 8 is a plot of the amplitude versus time of a pair of waveforms exhibiting alternate polarity sets according to the present invention.

[0034]FIG. 9 is a plot of the amplitude versus time for a sequence of four waveforms according to the present invention.

[0035]FIG. 10 is a plot of the amplitude versus time for another sequence of four waveforms according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0036] Referring to FIG. 1, a metal detector 10 according to the present invention is shown. In the preferred embodiment of the invention, the metal detector 10 is particularly adapted for hand-held use, such as by hobbyists. The detector is used to search for metal objects 9 in a target volume 11 that is typically ground but may be any volume. Since the detector is typically used outdoors to search for metal objects away from independent sources of power, the detector also incorporates a battery 20 to power the detector. While the detector 10 may be used with any power source without departing from the principles of the invention, it provides high energy efficiency which is particularly desirable in a battery powered device.

[0037] The metal detector 10 includes a search head 12, a signal generator 14, and a sampling circuit 16. The search head is maintained in close proximity to the ground 11. The signal generator 14 generates interrogating signals for producing eddy currents in the metal objects 9. The interrogating signals typically have a frequency on the order of 10 kHz. The interrogating signals are transmitted to the metal objects through the search head 12, particularly, a transmit coil 22 therein. Eddy currents are produced in the target in response to the interrogating signals.

[0038] The transmit coil has a plurality of windings of wire and is typically circular in form. For a hand-held detector 10, the diameter of the transmit coil may be between about 3″ and 15″, with about 10″ being typical. The depth of detection typically ranges between about the diameter of the transmit coil to about 3′. This is because the transmit coil depends on induction to produce the eddy currents in the metal objects, and is too small to radiate significantly as an antenna at the interrogation frequency.

[0039] The eddy currents induce a response “R” in the search head 12, particularly a receive coil 24 therein. The receive coil 24 is coupled to the sampling circuit 16 which samples the response R. While the transmit and receive coil can be the same coil, it is preferable to employ at least the two separate coils 22 and 24 in a standard induction balance configuration. As a common alternative, a third coil may be used to null the effect of the transmit coil 22, wherein the coils 22 and 24 and the third coil are all concentric.

[0040] The response R has a magnitude and phase that are in general altered from the magnitude and phase of the original signal as a result of the (complex) impedance of the target. The impedance of the target has a real part that produces a (vector) component of the response that is in-phase with the interrogating frequency, and a so-called imaginary part that produces another component of the response that is in-quadrature, or 90 degrees out of phase with the interrogation frequency.

[0041] The real, or “Y” component of a given frequency specific response vector represents the effect of the conductivity of all of the material contributing to the response at the specific frequency, while the imaginary, or “X” component represents the effect at the frequency of the reactance of this material. For non-ferrous metal materials and salt-water, the resistive component of the response will be much greater than the reactive component; conversely, for ferrous metals and soil containing iron, the reactive component is larger than the resistive component. Typically, two synchronous detectors aligned to two different phases in quadrature are used to resolve the total frequency specific response into its Y and X components, or “channels,” each providing information about the target volume of ground. Particularly, the ratio of the X and Y component amplitudes of the two channels provides the phase shift of the response with respect to the transmitted or interrogating signal that is caused by the target.

[0042] In addition to the requirement to distinguish the response from ground, it is necessary to be able to distinguish one metal object from another, i.e., to distinguish one phase shift from another. Accomplishing both of these objectives generally requires two independent methods for detecting responses that are insensitive to ground.

[0043] In one conventional method for this purpose, termed “motion discrimination,” the user is required to keep the transmit coil in motion. The ground is not necessarily homogeneous, but both the permeability and the conductivity of the ground are relatively constant over a limited area, so that detected signals resulting from ground are relatively slowly varying compared to signals resulting from the much more localized metal objects; the metal objects cause changes in the detected signals that are perceived by the metal detector to have a relatively high frequency. Then, high or band-pass filtering may be used to filter out the low frequency portion of the detected signals corresponding to the ground.

[0044] In the other conventional method for detecting responses that are insensitive to ground, often referred to as ground balancing, a location on the ground is selected for calibrating the detector, and it is determined how to linearly combine the X and Y components of the response so that it is zero, or put another way, it is determined how much to rotate the X and Y coordinate system to align it with the phase angle of the ground so that, at the phase angle of the ground, the response is nulled. In practice, this can be achieved by varying the phase angles of respective synchronous demodulators so that the demodulators are insensitive to components with a phase equal to the phase angle of the ground.

[0045] The two methods may be combined to permit measuring of the phase angle of the response independent of the effects of ground. Preferably, the metal detector 10 provides two channels of data for which the effect of the ground is removed or nulled in accord with the principles illustrated below, and which may also be rendered to be additionally insensitive to ground according to either or both of the aforementioned conventional methods.

[0046] Turning to FIG. 2, the signal generator 14 produces an electrical signal 26 which is characterized by one or more “waveforms” 28 in time sequence, shown as 28 a, 28 b and 28 c. Each waveform 28 is in turn characterized by an associated “settling period” 30, during which the amplitude of the waveform is constant, followed by an “interrogating period” 32, during which the amplitude of the waveform changes continuously in a time-varying manner, such as is shown. The settling period 30 is selected to be at least a minimum time during which the response of the target to any previous excitations has or would have decayed a desired amount.

[0047] Preferably, the electrical signal 26 describes current flow through the transmit coil 22, the amplitude of each waveform 28 is substantially zero during the settling period, and the amplitude of the waveform during the interrogating period varies so that there is a wide range of rate of change of current flow during the excitation and a large number of harmonics. More particularly, during the respective interrogating period each waveform 28 preferably comprises one or more pulses 34, each pulse representing a variation in amplitude of the waveform defined by a half-sine wave (“half-sine”), examples of which are given in FIGS. 3A-3E. While half-sine pulses are rich in harmonics and provide a wide and continuous range of rate of change of current, other waveforms or pulses may be employed without departing from the principles of the invention.

[0048] A generalized current pulse according to the invention is defined as commencing from a first baseline value of the current, then changing in a time-varying manner until reaching a peak value (that can be either positive or negative) at which the rate of change of the current is zero, and then changing in a time-varying manner to a second baseline value of the current, where the absolute magnitude of the peak value is greater than that of either of the baseline values of the current. The first and second baseline values of the current can be established in respective quiescent periods during which the current is substantially constant at first and second quiescent values, respectively. Alternatively, the first and second baseline values can be established by, respectively, the termination or commencement of another pulse. Preferably, the two baseline values of the current are the same, but this is not necessary, and more preferably, the two baseline values of the current are zero.

[0049] Preferably, the current changes so that the time-varying rate is either always positive or always negative prior to reaching the peak value, and thereafter changes so that the time-varying rate is either always negative or always positive, respectively. More preferably, the current changes at the rate of a half-sine or a substantial approximation thereof which commences and terminates at a nodal point of phase.

[0050] For purposes of description herein, the single sinusoidal period of current flow through the transmit coil 22 of FIG. 3B is defined as comprising two half-sine pulses 34 a and 34 b. The pulse 34 a originates at the 0 degree nodal point of phase “N₁” and terminates at the 180 degree nodal point of phase “N₂”, while the pulse 34 b originates at the 180 degree nodal point of phase “N₂” and ends at the 360 degree nodal point of phase “N₃” of a sinusoid. An equally useful alternative characterization is that both pulses 34 a and 34 b originate and end at the same nodal points of phase of a sinusoid, where pulse 34 a has positive polarity and pulse 34 b has negative polarity. The pulses 34 a and 34 b are substantially continuously connected in time, i.e., the end of the pulse 34 a occurs at substantially the same time as the start of the pulse 34 b at the point “N₂.”

[0051] Turning to FIG. 4, to produce the two half-sine pulses 34 a and 34 b shown in FIG. 3B, the signal generator 14 produces the sinusoidal transmit voltage 36 as shown. From this example, it will be readily appreciated by persons of ordinary skill how to obtain any of the half-sine pulses 34 shown in FIGS. 3A-3E.

[0052] To produce the voltage 36, the signal generator 14 includes a controller 37 and a transmit voltage output circuit 38. The output circuit 38 produces the electronic signal 26, here particularly the voltage shown in FIG. 4 to produce two half-sine pulses 34 having the current shown in FIG. 4. A power supply 40 deriving power from, e.g., the battery 20, powers the output circuit 38. Sometime during the settling period for a waveform, the controller 37 closes an electronic switch S1 between the power supply 40 and a capacitor C to charge the capacitor C. At a time t₁ representing the commencement of the pulse, the controller closes another electronic switch S2 to complete a resonant tank circuit formed of the capacitor C and the transmit coil 22. The switch S2 remains closed over one full cycle of oscillation, at the end of which, at time t₂, the current through the transmit coil is zero and the voltage across the coil is maximum. At the time t₂, the capacitor is nearly recharged by back emf from the transmit coil, and the controller 37 opens the switch S2. It is an outstanding advantage of the metal detector 10, as compared to the pulse induction or time domain detectors generally, that there is not generally a need for the damping resistor across the transmit coil that was used to critically damp prior art current flow in the search head.

[0053] Turning to FIG. 6, two pulses 34 (i.e., 34 a, 34 b) forming a full single period of a sine wave are shown. In this example, the two pulses 34 form a 200 microsecond interrogating period 32 (as defined in FIG. 2) for a single waveform 28, and were preceded by a settling period 30 (not shown). Also shown are the responses of various target materials to the pulses 34 during the time of excitation, wherein the length of the settling period 30 was chosen so that all previous responses of the target materials to previous excitations had decayed to zero. Specifically, FIG. 6 plots a response R₁ for ground mineralization (ferrite), R₂ for a steel bottlecap, and R₃ for a large block of aluminum, or a large coin such as a quarter.

[0054] The curve R₁ for ferrite crosses the time axis at points “A,” and “A₂”, i.e., the amplitude R₁ is zero, at approximately 90 degrees with respect to each of the pulses 34 a and 34 b, i.e., when the magnitude of the current is maximum and its slope is zero. The points “A” are “natural ground balanced” points, because the ferrite contribution to the detector's response at these points is zero, because the current is momentarily unchanging with time. The natural ground balanced points “A” are not precisely located, as they vary somewhat depending on the viscous magnetic remanence of the soil; moreover, where the soil includes conductive materials aside from ferrite, these will also cause the location of the points “A” to move slightly in time. This problem has been solved before in the metal detector art by providing a “ground balance” control on the instrument for the user to adjust the location of the points “A” in time, and more recently by the provision of specialized circuitry in the detector providing automatic adjustment of the ground balance points. Any such means may be used in connection with the present invention as well.

[0055] The ferrite response R₁ is also equal and opposite, for the excitation shown in FIG. 6, at points “J” and “N.” Particularly, for the pulse 34 a, the ferrite response R₁ is equal and opposite at points “J₁” and “N₁,” and for the pulse 34 b, the ferrite response R₁ is equal and opposite at points “J₂” and “N₂.” Therefore, for each pulse, the sum of R₁ measured at time “J,” and R₁ measured at time “N,” is zero, producing another natural ground balanced “point” “J+N.” Like the points “A,” the points “J” and “N” vary somewhat in time around points that are about 60 degrees on the leading edges of the pulses 34. It may be noted that additional ground balanced points may be derived from “A” and “J+N.” For example, the ratio A/(J+N), or the reciprocal, produces another point that is unchanging in the presence of ferrite ground. Any other combination or function of the points “A” and “J+N” would also be insensitive to ferrite ground.

[0056] Preferably, the response R of the metal detector to the current pulse is sampled substantially at the peak of the pulse, where the rate of change of the current is zero, to obtain a ground balanced point, and at least two samples are taken respectively on either side of the peak, where the slope of the pulse, i.e., di/dt, is of opposite sign for the two samples. As mentioned above, preferably, the current in a pulse according to the invention changes so that the time-varying rate, i.e., di/dt, is either always positive or always negative prior to reaching a peak value (which can be positive or negative) at which the rate of change of the current is zero, and thereafter changes so that the time-varying rate is either always negative or always positive, respectively. It is also desired that a range of di/dt is available before and after the peak current is reached. While a half-sine current pulse commencing and terminating at nodal points of phase is one example of such a pulse, a half-circular or a trapezoidal current pulse shape provide other examples. However, a square pulse is not ideal because, essentially, the entire duration of the pulse is at the peak value, and it is impractical to sample during the very small durations of the rising and falling edges of the pulse that remain to obtain the remaining samples.

[0057] With one ground balanced point obtained at substantially the peak of the current pulse, and a second ground balanced point obtained from the sampled values on either side of the peak, the effect of the ground can be removed or nulled in both of the aforementioned channels of the response R.

[0058] According to the principles above, a target volume of ground that lacks metal objects is used to calibrate the detector 10. The target is interrogated to produce a response “R” in the search coil indicative of ferrite in the ground. Points “A” and “J+N” as described above may be identified as natural ground balanced points. In addition or in the alternative, other ground balanced points may be derived from the response “R.” Referring to FIG. 7, samples P_(n) (n=13) are taken periodically during the time of excitation of a target volume of ferritic ground by a single half-sine pulse 34. The samples could be taken at any alternative times. The samples P of the response R₁ are not ground balanced. However, it is always the case that two or more sample points may be combined to produce a ground balanced (or null) result. For example, any two points P_(a1) and P_(a2), e.g., P₃ and P₄, or P₃ and P₈, or any other combination or group of two points, can be made to produce a null result by multiplying one of them by the negative reciprocal of the other and forming a sum.

[0059] The two points could alternatively be made to produce a null result by use of any number of alternative functions. For example, the magnitudes of the response “R₁” at the points “P_(n)” may be transformed in any desired manner for combining as described above, such as by taking their roots. Similarly, any number of sample points greater than two may be made to produce a null result by use of an appropriate function, the parameters of which are adjusted appropriately. A most simple example for three sample points P₁, P₂ and P₃ is to form the sum (P₂+P₃), multiply P₁ by the negative reciprocal of (P₂+P₃), and add these two results together to obtain a single ground balanced point. As may be readily apparent, more functions may be constructed the more points are being combined.

[0060] A minimum of three sample points “P” of the response “R” are required to obtain two ground balanced points. For example, the three sample points A, P₁, and P₂, yield the two ground balanced points “A” and “P₁−P₂(P₁/P₂).” Preferably, the three sample points are obtained either within a maximum of about 500 microseconds from the start of the interrogation period, and even more preferably within a maximum of about 100 microseconds from the start of the interrogation period, or within a single pulse, i.e., between the time that the pulse first commences from a quiescent value of the current during the settling period, first reaches a peak (either positive or negative), and subsequently first returns to the quiescent value of the current, to minimize the effects of averaging.

[0061] While any duration for the settling period may be provided according to the principles of the invention, preferably the minimum duration of the settling period is about 20 microseconds to ensure that viscous magnetic remanence has at least substantially decayed for small targets, although this period may more preferably be limited to a minimum of about 200 microseconds for many targets, or larger targets.

[0062] Once it is calibrated, the detector 10 may be used to interrogate another target volume of ground that may contain a metal object, wherein it is assumed that the ground in the two target volumes is for practical purposes the same. The same excitations are used that were used for calibration, and the same mathematical manipulation of the sample points is employed as well so that the effect of the ground is substantially nulled out. Thence, any residual response at the ground balanced points, either natural or artificial, is due to the metal object. With at least two ground balanced points obtained within about 100 microseconds of the beginning of the excitation, there can be sufficient information to identify the metal object, and employing additional data points may provide additional information for ensuring accurate identification.

[0063] The sampling circuit 16 typically samples the response “R” of the detector 10 within “windows” that are on the order of 10 microseconds centered respectively on the response points of interest, e.g., “A,” “J,” “N,” and any P_(n). With additional reference to FIG. 1, the detector also includes a processor 50, such as a DSP, that produces the ground balanced points, which may be any of “A,” “J+N,” f₁(A), f₂(J+N), f₃(A, (J+N)), and f₄ (P_(a)), where f₁, f₂, and f₃ can be any functions, and where f₄ is a function meeting the nulling conditions specified above. These ground balanced points are indicative of the metal object, and the processor 50 may compare them to pre-existing data in a look-up table 52 to identify the metal object. The detector 10 further typically includes a display device 54, such as an LCD, for reporting this identification to the user.

[0064] A sequence of the waveforms 28 of alternating polarity may be used to average out the effect of the earth's magnetic field. Referring to FIG. 8, one or more pairs of waveforms 28 are provided so that one waveform 28 a of a pair has a first sequence of pulses 34 defining a first set of polarities (or “polarity set”), and another waveform 28 b of the pair has a second sequence of pulses defining a set of polarities opposite those of the first polarity set. For example, pulses 34 a and 34 b of the waveform 28 a define a set “SET1” of polarities that is first positive and then negative (i.e., +, −). That is, the pulse 34 a represents a variation in the amplitude of the waveform 28 a that is above the level of the settling period 30 a, and the pulse 34 b represents a variation in the amplitude of the waveform 28 a that is below the level of the settling period 30 a. For the adjacent waveform 28 b, pulses 34 c and 34 d define a set “SET2” of polarities that is first negative and then positive (i.e., −, +) and is therefore opposite the set SET1. There may be additional pairs of waveforms, as shown, and the pulses 34 of any one waveform 28 may be arranged in any type, order or combination, provided that the same type, order and combination of pulses is provided in opposite polarity in the other waveform of the pair.

[0065] In the examples of specific waveforms 28 provided so far, the amplitude and period of the half-sine pulses has been the same for all the pulses, and the settling period for all waveforms 28 has also been shown to be a constant amount. However, as suggested by FIG. 2, none of these conditions is necessary to practice the invention. Further, it is believed to be advantageous to vary at least the period of the half-sine pulses and the settling periods of the waveforms.

[0066] Turning to FIG. 9, a sequence of four waveforms 28 a, 28 b, 28 c and 28 d is shown. The waveforms 28 a and 28 b form one pair and the waveforms 28 c and 28 d form another, alternating pair according to the methodology just discussed above.

[0067] The waveform 28 a has a settling period 30 a preceding an interrogating period 32 a consisting of a single half-sine pulse 34 a having a period “T_(a)/2.” The sinusoidal period T_(a) is about 67 microseconds, corresponding to a sinusoidal frequency of about 15 kHz. The subsequent waveform 28 b has a settling period 30 b that has a duration of about 5T_(a)/2, and an interrogating period 32 b consisting of two consecutive half-sine pulses 34 b ₁ and 34 b ₂. The pulse 34 b ₁ has a polarity opposite to that of the pulse 34 a and has a period “T_(b1)/2” which corresponds to a sinusoidal frequency of about 5 kHz. The adjacent pulse 34 b ₂ has the same polarity and period as the pulse 34 a. The settling period 30 a is about 8T_(a)/2.

[0068] The sampling circuit 16 in this embodiment samples the response R of the metal detector 10 at the peaks in magnitude of the pulses, such as at “A_(34c), “A_(3d1),” and “A_(34d2)” of the respective pulses 34 c of the waveform 28 c and the pulses 34 d ₁ and 34 d ₂ of the waveform 28 d. As with the points “A” discussed above in connection with FIG. 6, the points “A_(34c), “A_(34d1),” and “A_(34d2),” the corresponding point “A_(34a),” of the waveform 28 a, and the corresponding points “A_(34b1)” and “A_(34b2)” of the waveform 28 b are natural ground balanced points; accordingly, the effect of ground mineralization on these samples is substantially zero.

[0069] Forming a ratio of the response “R” of the metal detector 10 at points “A_(34a)” and “A_(34b1),” or equivalently “A_(34c)” and “A_(34d1),” and comparing the result with the look-up table 52 permits characterizing metal in the target where the metal's response to interrogation is frequency dependent, because the half-sine pulses 34 a and 34 c have different periods or sinusoidal frequencies than the half-sine pulses 34 b ₁ and 34 d ₁. The points “A_(34b2)” and “A_(34d2),” corresponding to the relatively high frequency half-sine pulses 34 b ₂ and 34 d ₂, are useful to discern whether the metal in the target, or part of the metal, is iron. The response “R” at these points is affected by the immediately preceding relatively low frequency half-sine pulses 34 b ₁ and 34 d ₁ because there is no settling time therebetween, so the response of metal objects has not had time to decay away. This provides a different response than if a settling time had been provided. Similarly, a settling time could be used between the pulses 34 b ₁ and 34 b ₂, or 34 d ₁ and 34 d ₂, that is less than a sufficient amount for the metal object's response to the pulses to be independent of the effect of the preceding pulse to provide a different response and, therefore, more information about metal object

[0070] Turning to FIG. 10, another sequence of four waveforms is shown, providing an example of asymmetric or “multi-frequency” current pulses. Pulse 34 e ₁ has a duration t₁ to t₂, 34 e ₂ has a duration from t₂ to t₃, 34 f ₁ (which is equal and opposite to 34 e ₁) has a duration t₅ to t₆, and 34 f ₂ (which is equal and opposite to 34 e ₂), has a duration t₄ to t₅. These four pulses (corresponding to waveforms 28 e and 28 f) form a set of pulses SET3, where a corresponding set of pulses SET4 (corresponding to waveforms 28 g and 28 h) may also be provided that is equal and opposite to SET3, so that similar sampling for each set provides for canceling the effect of the earth's magnetic field.

[0071] Sampling of the response “R” of the metal detector to the pulses of SET3 may be done, e.g., as follows: data may be acquired during the time t₁ to t₂, corresponding to the response “R” to the relatively low frequency pulse 34 e ₁. Data may also be acquired during the time t₂ to t₃, corresponding to the response “R” to the relatively high frequency pulse 34 e ₂. Since the pulse 34 e ₂ follows immediately the pulse 34 e ₁, the response “R” during the period t₂ to t₃ will be affected by the pulse 34 e ₁, and this may provide additional information about metal objects in the target, and/or about the ground, than could have been obtained by the use of either pulse by itself.

[0072] One natural ground balanced point occurs at “A_(34e),” which may be utilized as described above. The ground balanced point “A_(34e)” corresponds to the relatively “low frequency” pulse 34 e ₁. A similar ground balanced point “A_(34f)” corresponds to the relatively “high frequency” pulse 34 f ₂.

[0073] While the foregoing discussion has related to sampling the response to an excitation during the excitation, it was also noted that sampling has typically been done in the prior art pulse induction detectors after excitation. While this strategy has the serious problem of sensitivity mentioned previously, using the strategy in conjunction with the present invention solves that problem. Accordingly, in addition to sampling during excitation, the present invention contemplates sampling after excitation as well, to any desired degree, to obtain still more information about metal objects in the target volume of ground, or about the ground itself, that may be helpful to identify the metal objects. For example, for the waveform of FIG. 10, sampling of the response “R” of the metal detector 10 may also be done during the time t₃ to t₄, during the settling period preceding the pulses 34 f ₂ and 34 f ₁.

[0074] It is to be recognized that, while a particular pulse induction metal detector having high energy efficiency and sensitivity has been shown and described as preferred, other configurations and methods could be utilized, in addition to those already mentioned, without departing from the principles of the invention. For example, half-sine current pulses have been described herein as being preferred forms of the excitation during the interrogation period of the waveform 28 for a number of reasons, including that they provide for a natural LC resonance in the search coil which is energy efficient, because they are rich in harmonics, and because they provide a wide range of rate of change of current. However, it is important to appreciate that any time-varying excitation may be used in accordance with the principles of the present invention, including what is known in the art as a continuous waveform, and frequency domain analysis techniques may be used in addition to time domain techniques or in the alternative.

[0075] The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions to exclude equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow. 

1. In a metal detector having a search coil, a source of current flow in the search coil for interrogating a target volume of ground, and a detecting circuit for receiving a response to said interrogating, a method for detecting one or more unknown metal objects in the target volume of ground and for discriminating the metal objects from the ground, comprising: (I) calibrating the metal detector with a calibrating volume of ground that is the same as or similar to the target volume of ground but in which the unknown metal objects are substantially not present, said step of calibrating including the steps of (a) providing that said current flow changes from an initial quiescent magnitude in a predetermined time-varying manner, (b) sampling the response of the detecting circuit to said step (a) at least three distinct times within a sampling period to produce a respective three calibration data points, and (c) combining at least two of said three calibration data points in such manner as to produce a null result; and (II) interrogating the target volume of ground, said step of interrogating including the steps of (d) providing a first quiescent period during which said current flow is substantially unchanging from a first quiescent magnitude, (e) thereafter changing the amount of said current flow from said first quiescent magnitude in the manner of step (a), (f) providing a second quiescent period terminating step (e) during which said current flow is substantially unchanging from a second quiescent magnitude; (g) sampling the response of the detecting circuit to said step (e) at least said three distinct times to produce a respective three target data points, and (h) combining the target data points in the manner of step (c) to produce at least one ground balanced target data point for comparison with preexisting data.
 2. The method of claim 1, wherein step (h) includes producing two ground balanced target data points from sampling at three distinct times.
 3. The method of claim 1, wherein, in said step (a), said time-varying manner of changing the amount of said current flow includes producing one or more distinct pulses defined by said current flow changing from a first value of said quiescent magnitude so that the rate of change of the current is always one of positive and negative until reaching a peak value at which the rate of change is zero, and thereafter changing from said peak value to a second value of said quiescent magnitude so that the rate of change of the current is always a corresponding one of, respectively, negative and positive.
 4. The method of claim 1, wherein one of said at least three distinct times in said sampling step (g) is during the time said current flow changes from said first value of said quiescent magnitude, one of said at least three distinct times is during the time said current is at said peak value, and one of said at least three distinct times is during the time said current flow changes from said peak value to said second value of said quiescent magnitude.
 5. The method of claim 2, wherein one of said three distinct times in said sampling step (g) is during the time said current flow changes from said first value of said quiescent magnitude, one of said three distinct times is during the time said current is at said peak value, and one of said three distinct times is during the time said current flow changes from said peak value to said second value of said quiescent magnitude, and wherein step (h) includes producing at least two ground balanced target data points from said sampling at said three distinct times.
 6. The method of claim 1, wherein said time-varying manner of changing the current flow in step (a) produces one or more pulses, and wherein in said step (g) said sampling does not include sampling the response during said second quiescent period.
 7. The method of claim 1, wherein said time-varying manner of changing the current flow in step (a) produces one or more pulses, and wherein in said step (g) said sampling includes sampling the response during said second quiescent period.
 8. The method of claim 4, wherein said time-varying manner of changing the current flow in step (a) produces one or more pulses, and wherein in said step (g) said sampling includes sampling the response during said second quiescent period.
 9. The method of claim 1, wherein the duration of said first quiescent period is at least about 20 microseconds and wherein the duration of said sampling period is no more than about 500 microseconds.
 10. The method of claim 7, wherein the duration of said first quiescent period is at least about 200 microseconds and wherein the duration of said sampling period is no more than about 100 microseconds.
 11. The method of claim 1, wherein said step (c) includes determining the parameters of a predetermined function for producing said null result, and wherein step (h) includes employing the parameters determined in step (c) in said predetermined function to produce said at least two ground balanced target data points.
 12. The method of claim 1, wherein said current flow is substantially zero during said first and second quiescent periods.
 13. The method of claim 4, wherein said current flow is substantially zero during said first and second quiescent periods.
 14. The method of claim 5, wherein said current flow is substantially zero during said first and second quiescent periods.
 15. The method of claim 13, wherein said pulses are half-sines.
 16. The method of claim 14, wherein said pulses are half-sines.
 17. A high efficiency hand-held metal detector for detecting a metal object, comprising: a coil; a source of current flow in said coil, said source adapted to produce one or more half-sine pulses of current in said coil; a detecting circuit for detecting a response to said current pulses; and a sampling circuit for sampling said response at predetermined times at least during the time of one of said pulses.
 18. The apparatus of claim 17, wherein said sampling circuit is adapted for sampling said response during the time of one of said pulses and during a quiescent period subsequent to said pulses during which said current flow is substantially unchanging from a quiescent magnitude.
 19. The apparatus of claim 18, wherein said quiescent magnitude is substantially zero.
 20. The apparatus of claim 17, further comprising a battery for providing power to the metal detector.
 21. The apparatus of claim 17, wherein said sampling circuit is adapted to sample at least one of said pulses at least three times, at least one time during the rising portion of the pulse, at least one time at the peak of the pulse, and at least one time during the falling portion of the pulse.
 22. The apparatus of claim 20, further comprising a processing circuit adapted to discern a first ground balanced point corresponding to said sampling at said peak, and to compute a second ground balanced point corresponding to said sampling at said rising and falling portions of the pulse.
 23. The apparatus of claim 17, wherein said source of current flow is adapted to produce a quiescent current flow of substantially zero magnitude preceding at least one of said pulses and following at least one of said pulses. 